"If your nations do not want to see America moderate its commitment to this alliance, each of your capitals needs to show support for our common defence," he said in a speech in Brussels.

Kaljulaid said this was not an unrealistic demand. "There is the legitimate expectation of the Americans that the Europeans spend more money on defence," she told Die Welt.

Following pressure from the US in 2014, NATO partners set the goal of increasing their defence spending to at least 2% of their gross domestic product within a decade. Only five nations meet that requirement today: the US, the UK, Greece, Poland, and Estonia.